Re: RS485 is bidirectional does it mean it is fullduplex?



"Paul E. Bennett" <peb@xxxxxxxxxxxxxxxxxx> wrote:
>Floyd L. Davidson wrote:
>>>Absolutely. Connecting to local ground at both ends of the run is a recipe
>>>for HUGE ground currents. I've seen (very) melted ground wires. It is
>>>indeed a safety hazard.
>>
>> Bullshit.
>
>When you deal with the MEGA-AMP power world you will get to see how much of

And since we are *not* dealing with a power distribution system,
your point seems to be less than appropriate.

>a safety hazard connecting the screens at both ends is. Consider that,
>under certain circumstances there may exist 75 to 80 Volts between the two
>earths and that the current source is huge by comparison to the screen
>current carrying capacity. If we assume that the run of cable has a screen
>resistance of about 1.5 Ohms (a reasonable figure for quite a 150m run of
>decent signal cable) then 80V * 80V / 1.5 Ohms gives you a power
>dissipation of 4.267kW along the length of the screen. So, you had better
>limit the current flow in the screen in some manner or you fry the signal
>cable.
>
>>>See my other post re isolated comms interfaces.
>>
>> Why not do some real research, and find out how it is actually
>> done.
>
>Many of us have and employ this sort of consideration in our daily practice.

Reality check time: telephone cables all across the country and
around the world are grounded every 3000 or 6000 feet.

--
Floyd L. Davidson <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@xxxxxxxxxx
.